Cold cathode fluorescent lamps (CCFLs) are used in large-scale display panels for backlight sources thereof. However, it is disadvantageous of using CCFLs in display panels, since a display panel is powered by DC power source while CCFL is driven by AC (alternating current) power. It is thus needed DC-to-AC converter in a display module to convert DC supply voltage to AC voltage for driving the CCFLs thereof, and the cost and volume of the circuit for the display are increased accordingly. Due to driven by DC power, light-emitting diodes (LEDs) attract more and more attentions for light sources of display panels.
FIG. 1 shows a conventional driver circuit 100 for a DC light source utilizing LEDs as the lamp elements thereof, which comprises a boost or buck DC-to-DC converter 102 having an input VIN connected to an input node 104, an input SW connected to a node 106, an input SHON receiving a clock 108, a ground terminal GND connected to ground, and a feedback input FB receiving a feedback current IFB, a capacitor C1 connected between the input node 104 and ground, an inductor L1 connected between the nodes 104 and 106, a diode D1 connected between the node 106 and an output node 110, a capacitor C2 connected between the output node 110 and ground, a resistor R1 connected between the feedback input FB and ground, and a string of light-emitting diodes LED1, LED2 and LED3 connected between the output node 110 and feedback input FB. The input voltage Vin1 connected to the input node 14 is converted by the converter 102 to generate a voltage Vin2 on the node 106 to supply for the lamps LED1, LED2, and LED3 to light them up.
However, light source composed of LED string is currently applicable to small-scale display panels only, due to the fact that much larger number of LEDs connected in series is required in a large-scale panel for sufficiently illuminating the whole panel. Nevertheless, the more the LEDs are connected in series, the higher the start-up voltage is required. Typically, 3.5 V is increased more for the voltage when one more LED is connected in series. If several LEDs are alternatively connected in parallel to reduce the required start-up voltage, a non-uniform brightness may be presented on the LEDs owing to non-uniform currents flowing through the LEDs. Consequently, LEDs are only used for the backlight sources in small-scale display panels, but not in large-scale display panels.
Therefore, it is desired a driver circuit for driving LEDs, especially for large-scale display panels.